The present invention relates to the manufacture of glass optical waveguide preforms or blanks by the doped deposited silica process and particularly to an improvement in the known outside vapor phase oxidation (OVPO) process whereby more durable optical waveguide blanks for high numerical aperture graded-index glass waveguides can be made.
U.S. Pat. Nos. 3,823,995 and 3,826,560 describe the manufacture of glass optical waveguides from silica and doped silica of high purity by depositing concentric layers of these materials on suitable tubular or cylindrical substrates and working the layered structures into optical waveguide filaments. In the process known as the outside or OVPO process, an optical waveguide blank is built up on a rotating, substantially cylindrical bait rod or mandrel by depositing layers of pure or doped silica glass in soot form thereon, the glass soot being vitreous (amorphous) and being produced by the flame oxidation of combustible volatile compounds of silicon, boron, germanium, phosphorus or the like. After the soot layers have been deposited, the mandrel is removed from the soot preform and the preform is then dried to remove water introduced during the deposition process, consolidated to a clear glass tubular preform, and drawn into an optical waveguide filament. A number of patents describing the manufacture of doped silica optical waveguides by the OVPO process have issued, and the representative patents, in addition to those noted above, include U.S. Pat. Nos. 3,806,570, 3,868,170, 3,933,454, 4,125,388, 4,165,223 and 4,173,305.
In the outside process the first-deposited layers will form the central core of the waveguide after the mandrel is removed and the soot blank or preform consolidated and drawn into a waveguide filament. For graded-index waveguides, these layers must have a higher refractive index than subsequently deposited layers so that the radial refractive index profile of the core will conform to the desired parabolic form needed for low light pulse dispersion in the waveguide.
The requirement for high refractive index at the central core means that the first-deposited layers will contain higher levels of refractive-index-increasing dopants, and will ordinarily exhibit higher coefficients of thermal expansion, than the subsequently deposited layers of the core. The highest expansion, innermost layer of the preform immediately adjacent to the mandrel is exposed when the mandrel is withdrawn from the preform, and forms the inner surface of the longitudinal preform center hole. This layer is in tension at ambient temperatures, due to its high expansion coefficient with respect to the remaining layers of the preform, and promotes crack propagation into the body of the preform. Especially with preforms for high numerical aperture (NA) waveguides, containing highly doped, high expansion cores, there is a substantial risk of breakage from center hole tension during the manufacture and subsequent handling of the preform, particularly after the soot preform has been consolidated to a clear tubular glass preform.
U.K. Patent Specification No. 1,521,826 includes an explanation of the problem of center hole stress in tubular optical waveguide preforms, and suggests the use of P.sub.2 O.sub.5 in combination with GeO.sub.2 as core dopants which can alleviate the problem of excessive core thermal expansion. Another proposed solution, outlined in published U.K. Patent Application No. 2,004,863, involves the use, in the outer core layers of the preform, of a dopant which increases thermal expansion without increasing refractive index. An example of such a dopant is B.sub.2 O.sub.3.
Although this latter procedure had been successfully used to make relatively high-numerical-aperture, graded index optical waveguides with high bandwidth, having NA values on the order of about 0.2, still higher numerical aperture values are desirable for some waveguide applications. Unfortunately, the use of larger quantities of B.sub.2 O.sub.3 in the peripheral core and cladding to reduce stresses at the inner surface of the consolidated glass preform is accompanied by undesirable reductions in viscosity in the B.sub.2 O.sub.3 -containing glass, causing difficulties in implementing the preform collapse and filament drawing steps of manufacture.
It is therefore a principal object of the present invention to provide an alternative method for making preforms for high-numerical-aperture glass optical waveguides, useful alone or in combination with previous methods, which method greatly facilitates the manufacture of graded index optical waveguides having numerical aperture values greater than about 0.2.
It is a further object of the invention to provide a cylindrical optical waveguide soot preform made up of a plurality of layers of vitreous soot and incorporating a longitudinal center hole left by the removal of the starting member or mandrel which can be consolidated to a clear glass tubular preform exhibiting improved resistance to breakage during the subsequent steps of waveguide fabrication.
Other objects and advantages of the invention will become apparent from the following description thereof.